Conventionally, JP-2008-97413 A teaches a technique such that information about a prediction vehicle position of another vehicle and a road on which the other vehicle is driving is obtained via a wireless communication with the other vehicle, and a collision possibility on the other vehicle is calculated. In this technique, when an altitude of the road on which the other vehicle is running is different from an altitude of the road on which the subject vehicle is running, it is determined that there is no collision possibility since an intersection of two roads is a grade separated crossing.
However, in the technique described in JP-2008-97413 A, it is necessary to obtain a large amount of information from the other vehicle to the subject vehicle via the wireless communication such as a prediction vehicle position of the other vehicle and the road on which the other vehicle is travelling. Accordingly, a size of data transmitting and receiving between the subject vehicle and the other vehicle becomes large. Further, it is required for the other vehicle to be capable of transmitting the large amount of information. Thus, if the other vehicle cannot transmit the large amount of information, the above function does not work sufficiently.